I. Field of the Invention
The present invention relates to an ion flow modulator used in a photocopying machine.
II. Description of the Prior Art
Photocopying machines with an ion flow modulator have been conventionally proposed. The principle of operation of a photocopying machine of this type is given as follows. Light irradiates a document, and light (document image) reflected by the document is guided by an optical system to a photoconductive layer of an ion flow modulator to be described in detail later. The ion flow modulator has an array of a plurality of holes so as to cause ions to flow therethrough. More specifically, ions flow through the respective holes in accordance with intensities of light components irradiating the respective portions of the photoconductive layer. The ions passing through the holes charge a dielectric drum, thereby forming an electrostatic latent image corresponding to the document image. Toner is attracted to the latent image on the dielectric drum, and a toner image is transferred to a copy sheet, thus completing a copying cycle.
An illustrative representation of a conventional ion flow modulator (Japanese Patent Disclosure No. 56-35150) is shown in FIG. 1. Rectangular ion flow control electrodes 12 are formed on an insulating substrate 10 parallel to each other. Ion flow passage holes 14 are formed near end portions of the control electrodes 12, in the substrate 10 and in a common electrode 24 to be described later. A common photoconductive layer 16 is formed on the respective control electrodes 12, and a transparent electrode 18 is formed on the layer 16. A single resistance layer 20 common to the respective holes 14 of the control electrodes is formed at the other end portions of the control electrodes 12. A transparent electrode 22 is formed on the layer 20. A common electrode 24 is formed on the lower surface of the substrate 10. Power sources 26 and 28 are connected to the electrodes 18 and 22 to apply positive and negative voltages to the layers 16 and 22, respectively. These power sources are also connected to the electrode 24. A corona charger 30 as a means for generating an ion flow is arranged under the holes 14. The charger 30 comprises a corona discharge electrode 32 and a shield electrode 34. The electrode 32 is connected to a DC power source 36.
An ion flow generated from the electrode 32 flows through the holes 14 and reaches a dielectric drum (not shown). The number of ions flowing through the holes 14 is controlled by potentials of the electrodes 12 having the holes 14. More particularly, when ions generated from the corona discharge electrode are positive ions, the number of the ions flowing through the holes 14 is decreased by the positive potentials of the electrodes 12, and is increased by the negative potentials. In this manner, the latent image corresponding to the potentials at the electrodes 12 is formed on the dielectric drum (not shown). When light does not irradiate the photoconductive layer 16, the resistance of the photoconductive layer 16 is larger than that of the resistance layer 20, so that the potentials of the electrodes 12 are negative under the control of the power source 28 and the number of ions flowing through the holes is increased. However, when light irradiates the photoconductive layer 16, the resistance of the photoconductive layer 16 is decreased, and the potentials at the control electrodes 12 are controlled by the power source 26, thereby decreasing the number of ions passing through the holes. In this manner, the number of ions passing through control electrodes which receive light is small, but the number of ions flowing through control electrodes which do not receive light is large. Therefore, the latent image corresponding to the optical pattern obtained by the layer 16 is formed on the dielectric drum.
The above-described conventional ion flow modulator has the following problems. First, a stripe pattern is often formed on the copied image, or linear omission of the resultant image tends to occur. These events are based upon variations in thickness of the photoconductive layer. More specifically, potential control of each control electrode is performed by utilizing the resistance of the photoconductive layer along the direction of thickness thereof. When the thickness of the photoconductive layer on the respective control electrodes is nonuniform, the number of ions passing through the respective holes vary even if light uniformly irradiates the photoconductive layer, thereby degrading the copied image quality. In practice, it is very difficult to obtain a uniform thickness of the photoconductive layer. Second, when pinholes are formed in the photoconductive layer along the direction of thickness thereof, the photoconductive layer is subject to dielectric breakdown and DC current flows in the control electrode immediately under such pinholes in the photoconductive layer. In addition, since the holes are rendered conductive by the resistance layer, potentials at all control electrodes cannot be controlled. Third, since light from the document is incident on the photoconductive layer through the transparent electrodes, sensitivity is degraded.